Physics 271L Experiment 4

Construction of Voltmeter or Ammeter From a Galvanometer

Before designing a voltmeter or ammeter from a galvanometer it is
necessary to determine the resistance of the galvanometer,Rg,
and the value of the current which causes the galvanometer to read full scale, If. These values are
often stamped on the galvanometer by the manufacturer. We shall find Rgusing the half deflection method and the circuit in Fig. 1. BE VERY CAREFUL NOT TO PASS TOO LARGE A CURRENT THROUGH THE GALVANOMETER. Before plugging the line cord in, set R1= 25,000andR2= 1. Turn on the
current and adjustR1to
produce a full scale deflection on the galvanometer withS3open. Then closeS3and adjustR2until
you obtain a half-scale deflection. IfR1 >> R2
thenRgR2. If R1 is not much greater thanR2this approximation is not good. You
will get best results if the voltage divider is set to give maximum voltage to the circuit
and R1 >> R2to begin with. If the slide wire gets to end A and you still do not have full
scale current in the galvanometer TURN OFFthe current and start over with a reduced value of R1(8000 ). The full scale current of the galvanometer is
read directly from the ammeter whenS3is open. For this determination use the 15mAscale of theVOMfor the
ammeter.

Design of a Voltmeter

In addition to determining Rgand If you must also select a voltage V which you wish to cause a full-scale deflection. This is your choice, determined
by the intended use of the voltmeter. The only restriction of V is that it must be greater than or equal to If Rg (Why?)

The voltmeter is constructed by wiring a resistor R in series with the
galvanometer as shown in Fig. 2.

When the potential difference between the voltmeter terminals T1andT2 is V, the current through G must be If. Summing the voltage drops across R and Rg, in this
case, gives

1. Just as it was necessary to choose a full-scale voltage for the
voltmeter so it is necessary to select a current I which you
wish to cause a full-scale deflection. Again this choice is based on the intended use of
the ammeter and is limited only by the condition I > If. (Why?) The ammeter is constructed by wiring a resistor R, called a shunt, in parallel with the
galvanometer as shown in Fig. 3.

When the current I is
in the ammeter, you want If
as the current through the galvanometer and, by subtraction, I-If through R. Since the voltage
drop across Rg must be the same as the voltage
drop across R(Why?), the following equation holds.

If Rg =
(I-If)R or R = IfRg/(I - If)

2. Ayrton Shunt Method. Sometimes a shunt resistance needed is
too low for the resistances actually available (e.g. need 0.007, but only have 1 ). The lowest available resistance can be used as Rx, but another resistance, Ry, must be put in series with Rgto protect the galvanometer. Generally, if Rxis used,then Ryshould be: Ry = ([I/If] -1)Rx -Rg

a. Construct the circuit shown in Figure 1 to determineIfand Rg. After you have done so, do not do anything until your instructor verifies
your values are correct by watching you make your measurements again.

b. Next, construct the circuit in Figure 2 to convert the galvanometer to
a 0-20Vvoltmeter. Measure voltages of
about 4, 8, 12Vand compare the
readings on the constructed meter with those of the commercial voltmeter connected between
the same points. (See figure 2).

c. Convert the galvanometer to a 0-10.0mAammeter. Construct the circuit of Figure 3. Compare the readings for your
constructed ammeter to those of the commercial ammeter.

d. To construct a meter which will measure a current Iof1.0 A, follow the circuit in Fig. 4 and pick a value for Rxwhich is easily available, namely, a standard resistor for Rx= 1.0 . Now you calculate the value
of Ryon a decade box and place it in
series with your galvanometer. Place your meter in a reconstructed circuit of Figure 3.
Check the reading of the two meters. Set the decade box initially at 8000 and reduce gradually, comparing the currents on the two meters.

Questions:

1. How is a multirange voltmeter internally constructed (like the VOM you have been using)? Draw a diagram and make
calculations for 2 V, 20 V, 50 V, and 100V ranges, if If=1mA and Rg = 20 .

2. How is the internal resistance of the multivoltmeter changed as the
selected full-scale voltage V is
increased? Is this a desirable change, knowing an ideal voltmeter is to have very, very
high resistance?

3. How is a multirange ammeter constructed? Draw a diagram and make
calculations for 0.05 A, 0.10 A, 0.5 A,
and 1.0 A ranges, if If = 1mA and Rg
= 20 .

4. How is the internal resistance of a multirange ammeter changed as the
selected full-scale current I is
increased? Is this a desirable change, knowing that an ideal ammeter should have almost no
resistance?

5. What practical difficulties arise in constructing meters having large
current ranges? How are these difficulties helped with the Ayrton shunt?